This invention relates generally to circuit boards and sockets for mounting devices and modules thereon, and more specifically to techniques for mounting memory modules such as dual in-line memory modules (DIMMs).
It is well known to implement system (main) memory in computer systems, including personal computers and workstations, using memory modules having an industry-standard configuration. In short, a memory module for system memory typically includes a number of dynamic random access memory (DRAM) chips mounted on a small circuit board having edge contacts configured to engage complementary contacts in a socket. The module socket is mounted to a circuit board, which may be a main system board (motherboard), or a memory board that is itself mounted to a socket on the motherboard.
Recent industry-standard module configurations have included single in-line memory modules (SIMMs) and dual in-line memory modules (DIMMs). SIMMs have the memory chips on one side of the circuit board while DIMMs have memory chips on both sides of the circuit board. There are also memory modules for external cache memory using faster static random access memory (SRAM) chips rather than DRAM chips.
Although the memory chips themselves are subject to frequent changes, such as increases in capacity and speed, it is common practice to establish a number of standard mechanical configurations or form factors so that the number of different types of sockets remains relatively stable. In most desktop applications, the sockets are configured so that the DIMMs are perpendicular to the memory board or the motherboard. In laptop computer applications, where height above the motherboard is at a premium, the sockets are configured so that the DIMMs, when seated, are parallel to the motherboard
FIGS. 1A, 1B, and 1C are simplified top, bottom, and side views of a prior art DIMM 10. The designation of orientation will be with reference to a horizontal motherboard. In the particular configuration, the DIMMs are mounted horizontally (i.e., parallel to the motherboard). As can be seen, DIMM 10 includes a circuit board 12 on which are mounted memory chips. There are can be different numbers of chips on a DIMM (say two, four, or eight). A common configuration, shown here as a concrete example, has eight chips, here designated 15a-15d (top) and 15e-15h (bottom). The chips have pins 20, which are electrically connected to a set of upper edge contacts 25 and a set of lower edge contacts 27 on circuit board 12 by a series of conductive traces 30, only a few of which are shown. In accordance with standard multilayer board practice, some of the conductive traces are on one of the outer surfaces of the circuit board and some are on a buried layer.
The edge contacts on the DIMMs are often referred to as pins. The particular DIMM illustrated has 144 pins, with odd-numbered pins #1, #3, . . . #143 on top, and even-numbered pins #2, #4, . . . #144 on the bottom.
The upper pins are offset relative to the lower pins, so that pin #2 is between pins #1 and #3, and pin #143 is between pins #142 and #144, taken in the direction of the edge along which the pins are disposed. Other standard DIMM pin configurations include 112, 128, 144, 160, 168, and 200 pins.
The DIMM and its socket are keyed in order to prevent the DIMM from being inserted into the socket upside down and to prevent an incompatible DIMM from being inserted into the socket. Specifically, circuit board 12 is formed with a non-symmetrically located notch 35 that divides the contacts into short and long subsets (relative length along the edge). Thus, notch 35 divides upper edge contacts 25 into a short subset 25a and a long subset 25b, and divides lower edge contacts 27 into a short subset 25a and a long subset 25b.
FIGS. 2A, 2B, 2C, and 2D are simplified top, bottom, module-receiving side, and back side views of a prior art DIMM socket 40. The socket includes a socket body 45, formed with a module-receiving recess 47, and module latches 50a and 50b. A key 55 is formed on the module-receiving side of the socket body, and effectively divides the module-receiving recess is into short and long segments, designated 45a and 45b.
The socket includes a set of upper contacts 60 and a set of lower contacts 62. The upper and lower contacts on the socket are offset in a manner consistent with the offset between the upper and lower pins on the DIMMs. As in the case of notch 35 dividing the sets of contacts on the DIMM, key 55 divides the upper set of contacts into a short subset 60a and a long subset 60b, and divides the lower set of contacts into a short subset 62a and a long subset 62b.
The upper contacts include respective first ends (not shown) located along the upper surface of recess 47 and respective second ends on a lower surface of the socket and extending away from the back side. The first ends are configured to engage DIMM pins 25 while the second ends are configured to engage pads on the circuit board to which the socket is to be mounted. The lower contacts include respective first ends (not shown) located along the lower surface of recess 47 and respective second ends on a lower surface of the socket and extending away from the module-receiving side. The first ends are configured to engage DIMM pins 27 while the second ends are configured to engage a different set of pads on the circuit board to which the socket is to be mounted.
It is also known in the prior art to provide DIMM sockets as shown, but in a double-decker configuration configured to accommodate two DIMMs, one lying in a plane above the other. There would be two module-receiving recesses, each with its respective sets of upper and lower contacts. It is noted that where the number of pins becomes significantly larger than 200, as for example a double-decker socket configured for two 144-pin DIMMs, the contact density would likely necessitate the use of ball grid array (BGA) technology rather than the surface mount technology illustrated for the case of a total of 144 contacts.
FIG. 3 is a simplified top view of a prior art configuration with a pair of prior art DIMMS 10a and 10b disposed in respective prior art sockets 40a and 40b on a motherboard 70. The sockets are spaced as close together as is practical, given the space constraints of the specific application, namely a laptop computer. The arrangement shown is certainly serviceable, but the inventors have recognized a number of potential shortcomings. First, to the extent that there are situations with tight timing constraints, signal routing on the motherboard requires closely matched interconnections for a given signal to reach corresponding pins on the two DIMMs at the same time. Further, there may be circumstances where it would be desired to upgrade using a higher-capacity DIMM. It is likely that the chips on the higher-capacity DIMM would be longer in the dimension perpendicular to the edge of the DIMM circuit board with the edge contact, thereby necessitating a larger circuit board along that dimension (the DIMM would still have the same edge contact configuration). This capability could only be accommodated by increasing the spacing of the sockets, which is at odds with the design goal of minimizing the usage of scarce motherboard area.